Rock bolt with elongating section

ABSTRACT

A rock bolt device that includes a non-anchoring elongation section that allows elongation of the rock bolt device under tensile load to accommodate rock movement after installation of the rock bolt device.

RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application Ser. No. 62/399,899 filed on Sep. 26, 2016, and entitled “ROCK BOLT WITH ELONGATING SECTION”, the entire contents of which are incorporated by reference herein.

FIELD

The invention relates to rock stabilization, including in relation to a rock bolt, stabilized rock including an installed rock bolt and a method of stabilizing rock using a rock bolt, for example in a mine or other rock excavation.

BACKGROUND

Rock bolts are long bolt-type devices used to stabilize rock in rock excavation settings, such as in mines, tunnels or other rock excavations. When rock is excavated a pressure differential is created between the evacuated space of the excavation and the normal internal pressure of the rock. In deep mines or tunnels the pressure differential may be very large and the rock will tend to expand and move into the evacuated space as a result of the pressure differential, which can lead to rock failure at the rock surface of the evacuated space. Rock bolts are installed in the rock to transfer the load of rock near an exposed rock surface to a location deeper into the rock away from the surface. Rock bolts may be several feet long to transfer the load to rock located a significant distance away from the exposed surface of the rock that may retain better integrity than rock near the exposed surface. As the rock bolt transfers the load, the rock may become stabilized to a degree that migration of the rock into the evacuated space may be slowed significantly or even effectively halted to support the rock and prevent rock failure into the evacuated space. Some rock however, is particularly resistant to stabilization efforts and even with rock bolting may continue to move into the evacuated space and fail, sometimes shearing around the bolt head while leaving the rock bolt in place and other times by tensioning the rock bolt to a point where the bolt may have a material failure. A number of secondary supports, for example cable mesh between rock bolts, may be used to help provide additional support to stabilize rock in such situations, but there remains a significant need for high-performance rock bolts for rock that is difficult to stabilize.

SUMMARY

Disclosed is a rock bolt of a hollow core design to be secured with grout to anchor in and stabilize rock, and related stabilized rock structures and methods.

One aspect of this disclosure involves a rock bolt to be installed with grout to anchor in and stabilize rock, with the rock bolt comprising:

a bolt body having a longitudinal axis and a length along the longitudinal axis between a proximal end of the bolt body to be disposed outside of the rock and a distal end to be disposed in the rock when the rock bolt is installed to stabilize the rock;

a hollow core extending longitudinally through the bolt body and open adjacent the proximal end and the distal end to permit circulation of the grout through the hollow core and around an exterior of the bolt body to secure the bolt body in place with the grout when the rock bolt is installed to stabilize the rock;

the bolt body including at least the following longitudinal sections with relative longitudinal positioning moving along the longitudinal axis in a direction from the distal end toward the proximal end:

-   -   an anchor section comprising first exterior anchor projections         to project into and anchor in the grout disposed between an         exterior of the anchor section and the rock when the rock bolt         is installed to stabilize the rock;     -   a non-anchoring elongation section free of anchor projections on         an exterior of the non-anchoring elongation section and having         material properties for longitudinal elongation of the         elongation section under tensile load following installation to         accommodate rock movement in an axial direction of the         longitudinal axis;     -   a load absorption section comprising second exterior anchor         projections to project into and anchor in the grout disposed         between an exterior of the load absorption section and the rock         when the rock bolt is installed to stabilize the rock; and     -   a retention section to be disposed outside of the rock when the         rock bolt is installed to stabilize the rock;

a bearing plate engaged with the retention section of the bolt body and positionable along the retention section to contact a rock surface when the rock bolt is installed to stabilize the rock; and

wherein the bolt body has a tensile load capacity (preferably of at least 15 tons, more preferably of at least 20 tons and even more preferably of at least 25 tons; and often not larger than 50 tons, not larger than 35 tons or not larger than 30 tons) and has a pre-installation configuration (e.g, at-rest without externally applied load) prior to installation to stabilize the rock and a loaded configuration at the tensile load capacity, and the elongation section has a longitudinal length in the loaded configuration that is significantly longer (preferably at least 10 percent longer, more preferably and least 12 percent longer and even more preferably at least 15 percent longer; and often not more than 25 percent longer or not more than 20 percent longer) than in the pre-installation configuration.

Another aspect of this disclosure involves a stabilized rock adjacent a rock surface. The stabilized rock includes a rock bolt, such as of the disclosed rock bolt aspect, extending through the rock surface and into the rock and with the anchor section, elongation section and load absorption section disposed in the rock and the with the retention section and the bearing plate disposed outside of the rock and with grout disposed between the rock and the exterior of the anchor section and the exterior of the load absorption section engaging the first and second anchor projections.

Another aspect of this disclosure involves a method of stabilizing rock. The method includes:

disposing the bolt body of a rock bolt, such as of the disclosed rock bolt aspect, through a surface of the rock with the anchor section, elongation section and load absorption section disposed in the rock and the retention section disposed outside of the rock;

circulating grout through the hollow core and outside of the bolt body and around the exterior of the bolt body to engage the rock and the first and second anchor projections; and

positioning the bearing plate engaged with the retention section to contact the rock surface.

A number of feature refinements and additional features are applicable to the various aspects of this disclosure. These feature refinements and additional features may be used individually or in any combination within the subject matter of the first aspect or any other aspect of the disclosure. As such, each of the following features may be, but are not required to be, used with any other feature or combination of features of the any of the rock bolt aspect, stabilized rock aspect, method aspect or any other aspect of the disclosure.

The non-anchoring elongation section may be referred to as a “debonded” section, in that the section is intended to easily break free from, or debond from, the grout even when the grout extends around the exterior of the elongation section. This elongation section, without anchor projections that retain the section and the grout in a fixed relation and made of a material having a relatively low elastic modulus, will relatively easily shear free from the grout as tensile load in the elongation section increases and as the material properties of the elongation section permit the elongation section to elongate. In this way, the near-surface rock is permitted to move by a larger degree than with prior rock bolts that are not designed to provide such elongation, because of exterior anchor projections that retain the entire length of the rock bolt in a fixed relation with the rock and/or being constructed of only materials that are rigid and do not permit significant elongation as the bolt is subjected to increasing tensile loads. With the non-anchoring elongation section, rock adjacent the exposed surface of the rock is permitted to move by a greater amount into the evacuated space as tensile load increases in the bolt, which for some rocks may prevent or significantly delay rock failure around the bearing plate.

The longitudinal length of the elongation section under an identified tensile test load, which may be lower than the tensile load capacity, may be significantly longer (preferably at least 10 percent longer, more preferably and least 12 percent longer and even more preferably at least 15 percent longer; and often not more than 25 percent longer or not more than 20 percent longer) than in the pre-installation configuration. Specifying such an elongation proportion relative to a particular applied tensile test load permits the rock bolt to be designed to beneficially approximate rock characteristics of a variety of rock types in that elongation characteristics may be matched to a particular rock type to provide a tailored range of rock bolt elongation to beneficially better match with different rock properties. Such a tensile test load may be, for example, 15 tons, 20 tons, 25 tons or 30 tons.

The length of the bolt body (and for the entire bolt) may be any desired length for a particular application, for example at least 6 feet, at least 7 feet or at least 8 feet. Such a length may in some cases be not longer than 15 feet, not loner than 12 feet, not longer than 10 feet, not longer than 9 feet, or not longer than 8.5 feet. The length of the bolt body will also often be the length of the entire rock bolt, as proximal and distal ends of the bolt body will also typically be the proximal and distal ends of the entire bolt.

The relative lengths of the anchor section, elongation section, load absorption section and retention section may be varied for particular situations. The anchor section and load absorption sections should each be of sufficient length to provide the desired level of anchoring at the respective rock locations for absorption of the load through the load absorption section and transfer of the load to the rock to which the anchored section is anchored. The anchor section may for example have a length of at least 2 feet, at least 2.5 feet, at least 3 feet, and may sometimes be not larger than 5 feet, not larger than 4 feet or not larger than 3.5 feet, with about 3.25 feet (3 feet, 3 inches) being useful for some situations. The load absorption section may for example, have a length of at least 0.5 feet, at least 0.75 feet, at least 1 foot or at least 1.25 feet, and may sometimes be not larger than 2 feet, not larger than 1.5 feet or not larger than 1.25 feet. The non-anchoring elongation section should be long enough, in combination with material properties, to provide a desired level of elongation for the particular rock support application. For example, the elongation section may provide a total elongation of the length of the elongation section between the pre-installation configuration and the appropriate load configuration (either at tensile load capacity or at an identified tensile test load) of at least 4 inches, at least 5 inches or at least 6 inches, and which may sometimes be not larger than 1.25 feet, not larger than 1 foot, not larger than 10 inches or not larger than 8 inches. The retention section may provide sufficient length for engagement of the bearing plate and other attachment structures, such as a nut that may be disposed proximal of the bearing plate to retain the bearing plate at an appropriate longitudinal position against the rock surface when the rock bolt is installed. The retention section may have a length for example, of at least 0.5 foot, at least 0.75 foot or at least 1 foot, and may sometimes be not larger than 2 feet, not larger than 1.5 feet, not larger than 1.25 feet or not larger than 1 foot. As will be appreciated, the load absorption section and the retention section may be a single proximal end portion of the bolt body and the relative amount of that proximal end portion allocated to the load absorption portion and the retention portion, respectively, may be determined when installed to stabilize rock by the location of the rock surface in that proximal end portion, with the portion inside of the rock (disposed distal to the rock surface) being allocated to the load absorption section and the portion outside of the rock (disposed proximal of the rock surface) being allocated to the retention section, thereby providing some flexibility during use to adjust the relative lengths of the load absorption section and the retention section. For example, the proximal end portion of the rock bolt may be a threaded proximal end portion and the bearing plate may be disposed at a desired longitudinal location along that threaded proximal end portion to be retained against the rock surface at that longitudinal location by a nut threaded to a desired degree onto the proximal end of the bolt body. The longitudinal length of the threaded end portion distal of the bearing plate would then serve as the load absorption section with the threads acting as the second anchor projections. The first anchor projections may be any projection geometries that provide desired interaction with the grout when the rock bolt is installed. Such anchor projections may be, for example, in the form of threads, spaced ribs or knobs on the exterior of the anchor section.

Typical materials of construction for the rock bolt, including the bolt body, are metallic compositions and in particular steel compositions. The bolt body may be constructed of a single material of construction or one or more of the longitudinal sections may be constructed of a different material of construction than one or more of the other longitudinal sections. For example, the elongation section may be made of a more ductile steel alloy composition than one or both of the anchor section and the load absorption section. An elongation section made of a different material than the anchor section or the load absorption section may, for example, be connected with an adjacent longitudinal section through a connector. As another example, the elongation section may be of the same material of construction as one or both of the anchor section and the load absorption section, but with the first and/or second anchor projections having a geometry that in combination with the material properties may retain the respective longitudinal section bonded in a fixed relation relative to the rock even though the debonded elongation section may be free to elongate. In some configurations, the elongation section may have a maximum outside diameter of no larger than (and may be the same as or smaller than) a maximum outside diameter of the anchor section and/or a maximum outside diameter of the load absorption section. The maximum outside diameter of the anchor section and the maximum outside diameter of the load absorption section will typically be the outside diameter to the maximum extent of the first and second anchor protrusions, respectively. The exterior of the elongation section may preferably be a smooth, uniform surface of constant diameter. To assist debonding of the elongation section, and/or for added corrosion protection of the elongation section, an exterior of the elongation section may be covered with a plastic liner.

It will be appreciated that references to grout are to compositions that may be disposed between an exterior of the rock bolt and rock to bond the rock to the rock bolt, and in particular to the anchor section and the load absorption section. The term “grout” is not limiting with respect to compositional make-up, and includes for example compositions based primarily on inorganic components, as well as organic resin or resin-containing bonding compositions, and regardless of curing mechanism.

Reference to a ton is to a U.S. weight measure including 2000 pounds, although the same disclosure and principles apply as well to metric tonne measures as well.

The rock bolt and various other aspects of this disclosure will be further understood with reference to the FIGURE and to the claims.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE, including annotations, illustrate example features in relation to a rock bolt.

DETAILED DESCRIPTION

Reference is made to the FIGURE showing and describing features for an example rock bolt.

The foregoing discussion of the invention and different aspects thereof has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to only the form or forms specifically disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain best modes known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art. Although the description of the invention has included description of one or more possible embodiments and certain variations and modifications, other variations and modifications are within the scope of the invention, e.g., as may be within the skill and knowledge of those in the art after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate, disclaim or disavow any patentable subject matter. Furthermore, any feature described or claimed with respect to any disclosed variation may be combined in any combination with one or more of any other features of any other variation or variations, to the extent that the features are not necessarily technically compatible, and all such combinations are within the scope of the present invention. The description of a feature or features in a particular combination do not exclude the inclusion of an additional feature or features. Processing steps and sequencing are for illustration only, and such illustrations do not exclude inclusion of other steps or other sequencing of steps. Additional steps may be included between illustrated processing steps or before or after any illustrated processing step. Illustrated processing steps may include processing operations (e.g., sub-steps) in addition to particular processing operations illustrated or discussed with respect to the illustrated processing step.

The terms “comprising”, “containing”, “including” and “having”, and grammatical variations of those terms, are intended to be inclusive and nonlimiting in that the use of such terms indicates the presence of some condition or feature, but not to the exclusion of the presence also of any other condition or feature. The use of the terms “comprising”, “containing”, “including” and “having”, and grammatical variations of those terms in referring to the presence of one or more components, subcomponents or materials, also include and is intended to disclose the more specific embodiments in which the term “comprising”, “containing”, “including” or “having” (or the variation of such term) as the case may be, is replaced by any of the narrower terms “consisting essentially of” or “consisting of” or “consisting of only” (or the appropriate grammatical variation of such narrower terms). For example, a statement that some thing “comprises” a stated element or elements is also intended to include and disclose the more specific narrower embodiments of the thing “consisting essentially of” the stated element or elements, and the thing “consisting of” the stated element or elements. Examples of various features have been provided for purposes of illustration, and the terms “example”, “for example” and the like indicate illustrative examples that are not limiting and are not to be construed or interpreted as limiting a feature or features to any particular example. The term “at least” followed by a number (e.g., “at least one”) means that number or more than that number. The term “at least a portion” means all or a portion that is less than all. The term “at least a part” means all or a part that is less than all 

What is claimed is:
 1. A rock bolt to be installed with grout to anchor in and stabilize rock, the rock bolt comprising: a bolt body having a longitudinal axis and a length along the longitudinal axis between a proximal end of the bolt body to be disposed outside of the rock and a distal end to be disposed in the rock when the rock bolt is installed to stabilize the rock; a hollow core extending longitudinally through the bolt body and open adjacent the proximal end and the distal end to permit circulation of the grout through the hollow core and around an exterior of the bolt body to secure the bolt body in place with the grout when the rock bolt is installed to stabilize the rock; the bolt body including at least the following longitudinal sections with relative longitudinal positioning moving along the longitudinal axis in a direction from the distal end toward the proximal end: an anchor section comprising first exterior anchor projections to project into and anchor in the grout disposed between an exterior of the anchor section and the rock when the rock bolt is installed to stabilize the rock; a non-anchoring elongation section free of anchor projections on an exterior of the non-anchoring elongation section and having material properties for longitudinal elongation of the elongation section under tensile load following installation to accommodate rock movement in an axial direction of the longitudinal axis; a load absorption section comprising second exterior anchor projections to project into and anchor in the grout disposed between an exterior of the load absorption section and the rock when the rock bolt is installed to stabilize the rock; and a retention section to be disposed outside of the rock when the rock bolt is installed to stabilize the rock; a bearing plate engaged with the retention section of the bolt body and positionable along the retention section to contact a rock surface when the rock bolt is installed to stabilize the rock; and wherein the bolt body has a tensile load capacity of at least 25 tons and has a pre-installation configuration prior to installation to stabilize the rock and a loaded configuration at a tensile load of 25 tons, and the elongation section has a longitudinal length in the loaded configuration that is at least 15% longer than in the pre-installation configuration.
 2. A stabilized rock adjacent a rock surface, comprising a rock bolt according to claim 1 penetrating across the rock surface with the anchor section, elongation section and load absorption section disposed in the rock and with the retention section and the bearing plate disposed outside of the rock and with grout disposed between the rock and the exterior of the anchor section and the exterior of the load absorption section engaging the first and second anchor projections.
 3. A method of stabilizing rock, comprising: disposing the bolt body of the rock bolt of claim 1 across a surface of the rock with the anchor section, elongation section and load absorption section disposed in the rock and the retention section disposed outside of the rock; circulating grout through the hollow core and outside of the bolt body and around the exterior of the bolt body to engage the rock and the first and second anchor projections; and positioning the bearing plate engaged with the retention section to contact the rock surface. 